public class LazySingleton {
    private volatile static LazySingleton lazySingleton;
    private LazySingleton(){}
    public static LazySingleton getInstance(){
        if(lazySingleton == null){
            synchronized (LazySingleton.class) {
                if (lazySingleton == null) {
                    lazySingleton = new LazySingleton();
                }
            }
        }
        return lazySingleton;
    }
}

public class HungrySingleton {
    private final static HungrySingleton HUNGRY_SINGLETON;
    static {
        HUNGRY_SINGLETON = new HungrySingleton();
    }
    private HungrySingleton(){}

    public static HungrySingleton getInstance(){
        return HUNGRY_SINGLETON;
    }
}

public enum EnumSingleton {
    INSTANCE(new Object());
    private Object data;

    EnumSingleton(Object data) {
        this.data = data;
    }

    public static EnumSingleton getInstance(){
        return INSTANCE;
    }
}